$_letters: (
  'one',
  'two',
  'three',
  'four',
  'five',
  'six',
  'seven',
  'eight',
  'nine',
  'ten',
  'eleven',
  'twelve',
  'thirteen',
  'fourteen',
  'fifteen',
  'sixteen'
);

@function getLetterByNumber ($number) {
  @return nth($_letters, $number);
}

@function getNumberByLetter ($letter) {
  @return index($_letters, $letter);
}